"Little girls read books."
Translation:Les petites filles lisent des livres.
You are right, without any hint on the context, you could interpret this sentence two ways:
Little girls as the plural of A little girl => une petite fille -> de petites filles (des becomes de in front of an adjective)
Little girls [all] read books, as a generality => les petites filles (in general) read books (generalities use the definite article)
The definite plural article can also be used to refer to groups in a general sense. See here for more details: http://french.about.com/library/weekly/aa082401.htm
If you follow the links already posted to this discussion (e.g. this one http://french.about.com/library/weekly/aa082401.htm ) you can easily check that there is nothing wrong with duo's translation.
"les livres" = "the books" = specific books (the ones we gave them, for ex)
"les petites filles lisent des livres" is the plural of "les petites filles lisent un livre " = some books = more than one book.
Remember that "un/une" have a plural form: "des", whereas the English indefinite article "a" does not have a plural form.
Most adjectives are placed after the noun. Others are usually placed in front of them (most notably, the BAGS group: beauty, age, good and bad, size) . And a few can be placed either way. In that case, the meaning often changes depending on where you place it. See here for more details: http://french.about.com/od/grammar/a/adjectives_4.htm